• HTML
• CSS

HTML

<div class="box">DIV centrado con FLEXBOX</div>

CSS

body{
    width:100%;
    height:100vh;
    margin:0px;
    font-family:sans-serif;
    
    display:flex;
    justify-content:center;
    align-items:center;
}

.box{
    width:200px;
    height:200px;
    background:#76d7c4;
    padding:20px;
    border:4px solid #48c9b0;
    border-radius:10px;
    color:#0e6251;
}